1. Field of the Invention
This invention relates to a strong, lightweight tether or electro-optical-mechanical (EOM) cable. The tether is designed and primarily used for applications relating to airborne devices, specifically to attach the airborne device to the ground or a structure or vehicle on the ground while also performing other functions. Some of the functions include electrical power transmission either to the airborne device or from the airborne device if it includes a power generator. Electrical wires in the tether also allow for communication via electrical signals, although it is preferred to use the embedded optical fibers for communication. The tether may be used to send control commands to the airborne device, receive feedback from the airborne device, receive, detect or broadcast transmissions or collect surveillance and send it to the ground. The tether also exhibits other features including high strength and light weight, which make it ideal for use in aerospace applications, including electrodynamic tethers on spacecraft. The tether can also be used in other applications that require strength along with electrical and optical signal transmission.
2. Description of Related Art
Some tethers comprise only a single line. A single-line tether has limited functionality, and is prone to failure. Improvement has been shown by using multiple lines to produce the tether, which increases the strength as well as providing for the survival of the tether even if one line is broken (U.S. Pat. Nos. 6,116,544 and 6,431,497). In the referenced patents, the several lines comprising the tether are of the same material, providing an increase in strength and reliability but not in functionality. These prior art tethers are designed for conducting electricity and thus contain metal conductors. While these patents do not indicate specifically where the conductive wires are located, they do disclose metal and glass in the braided yarns. Further, the tether disclosed in U.S. Pat. No. 6,431,497 stretches a lot because the load bearing yarns are braided rather than being straight for the length of the tether.
For some applications, the materials required to function in a specific way are not strong enough to form a tether without reinforcement. These include optical fibers and electric wires. In order to make a tether capable of transmitting signals or electrical power, it is necessary to include strong fibers, so that the integrated tether is stronger than the optical fibers or electrical conductors. These strength members typically have a higher strain at failure than either electrical or fiber optic strands. To overcome this strain inequality, typical fiber optic cables are overloaded with high performance fibers to prevent strain and the accompanying conductor failure, as seen in U.S. Pat. No. 6,325,330. However, this approach fails to use the strength of the high performance fiber efficiently. The geometry of the current invention allows the strength members to be efficient in load bearing, while conductors elongate without failure using geometric form strain rather than material strain.